Enhanced performance of organic light-emitting diodes (OLEDs) and OLED-based photoluminescent sensing platforms by novel microstructures and device architectures

Organic light emitting diodes (OLEDs) have advanced dramatically since they exhibit great promise in various applications such as displays, solid-state lighting, and (bio)chemical sensing. In this dissertation, multiple approaches were employed to enhance the performance of OLEDs and OLED-based sensing platforms. Comprehensive investigations were conducted on electroluminescence (EL) spikes and tails in charge trapping guest-host OLEDs and their influence on OLED-based sensor performance. Novel microstructures and device architectures were developed to construct OLED sources with spectrally selective and enhanced emission. The peak emission wavelength of the multicolor microcavity devices with MoO3 as the HIL/spacer was tunable from 493 to 639 nm. The controlled microporous structures formed by polystyrene (PS):polyethylene glycol (PEG) was able to enhance the forward light extraction of the OLED by up to ~60%. The combination of the PtOEP:PS:PEG sensing film coupled with the multicolor microcavity OLEDs and the appropriate OPD, and the possibility to combine time- and intensity-domain analyses have shed light on the opportunities to realize simple, compact, potentially disposable sensors for the detection of O2, pH and other (bio)chemical analytes and parameters

Organic Light Emitting Devices

This book describes the state-of-the-art advancement in the field of organic electroluminescence contributed by many researchers with internationally established expertise in the field. It includes original contributions on the synthesis of suitable organic materials, fabrication of organic light emitting devices (OLEDs) and organic white light emitting devices (WOLEDs), characterization of these devices and some designs for optimal performance. All chapters are self-sufficient in presenting their contents. The cost effective chemical technology offers many exciting possibilities for OLEDs and organic solar cells (OSCs) to be futuristic solutions for lighting and power generation. A common flexible substrate can be used to fabricate OLEDs on one side facing a room and OSCs on the other side facing the sun. The device thus fabricated can generate power in the day time and light a room/house at night. The book covers developments on OLEDs, WOLEDs and briefly on OSCs as well

Improved performance of organic light-emitting diodes with MoO3 interlayer by oblique angle deposition

Cataloged from PDF version of article.We fabricated and demonstrated improved organic light emitting diodes (OLEDs) in a thin film architecture of indium tin oxide (ITO)/molybdenum trioxide (MoO3) (20 nm)/ N,N'-Di(naphth-2-yl)-N,N'-diphenyl-benzidine (NPB) (50 nm)/tris-(8-hydroxyquinoline) (Alq(3)) (70 nm)/Mg:Ag (200 nm) using an oblique angle deposition technique by which MoO3 was deposited at oblique angles (theta) with respect to the surface normal. It was found that, without sacrificing the power efficiency of the device, the device current efficiency and external quantum efficiency were significantly enhanced at an oblique deposition angle of theta = 60 degrees for MoO3. (C) 2011 Optical Society of Americ

Organic Electronic Devices for Sensing and Imaging Applications.

While electronic devices based on organic materials have an increasing presen-ce in display, lighting, and photovoltaic applications, sensor applications have been relatively unexplored. Organic semiconductors offer unique properties which make them particularly desirable for sensing and imaging, and which are impossible or difficult to realize in inorganic semiconductors. This work focuses on developing novel organic-based devices that function as imaging tools, including integrated surface plasmon-emitting devices, active organic devices integrated with scanning probe cantilevers, and tunable organic photodetectors. The fundamental physical mechanisms for excitonic energy coupling in thin film heterostructures, which enable the operation of these devices, are discussed in detail. Lower refractive index of organic compounds provides several electrical and optical advantages for device design, including large binding energy of excitons and relatively long lifetimes, which can facilitate efficiently coupling of exciton energy to surface plasmons at a nearby metal interface. This thesis describes how to utilize the strong excitonic energy coupling to surface plasmons in thin-film organic optoelectronic devices to transfer energy across a thick metal film, from electrically pumped molecular excitons on one side to molecular excitons on the opposite side. The observed exciton transfer mechanism can be applied to, for example, a solid-state electrically-pumped sensor with nanoscale resolution, or multi-modal imaging of nanomaterials and biological systems. Surface plasmon resonances in electrically-pumped devices can potentially be engineered to create sensitive biochemical reaction detectors, based on the inherent electrical sensitivity of the devices to the exciton decay rate (which in turn responds to nearby chemical species). In order to increase the spatial resolution of organic-based imaging devices, the fabrication of organic LEDs and organic photodetectors of submicron size on the tips of scanning probe cantilevers is described. The small working area is defined by focused ion beam milling, and the devices are used for simultaneous high-resolution optical imaging and topography measurements. An organic photodetector with spectral response tunable across the visible spec-trum is demonstrated by means of engineering the optical microcavity within the device. Such mechanically flexible and tunable photodetectors are expected to find applications in microfluidics and portable sensing devices, as well as colorimeters.Ph.D.Mechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/64724/1/lightan_1.pd

Oxygen, relative humidity, and interlayer related issues in organic electronics

Photoluminescence (PL)-based optical O2 sensors have applications in agricultural, medical and industrial areas. In this dissertation, All organic O2 sensors are realized by using organic light emitting diodes (OLEDs), porous O2 sensing film, and organic photo detectors (OPDs). The main idea is to create the voids in the O2 sensing film to have higher PL signal; and to modify the interfacial layer for organic photovoltaics (OPVs) to have higher sensitive photo detectors. This is a big step to have integrating small compact O2 monitoring platform. The modified porous sensing films also make it possible to have O2 and relative humidity (RH) monitored where less than 10% resolution in RH is obtained. This dual detection is important since those two parameters usually work together in the organic electronics device and food packaging. Furthermore, O2 monitoring can also be applied in the OLEDs or OPVs device, The active layer with the O2 sensitive material Pd octaethylporphyrin (PdOEP) can also be used as O2 sensing film. PL decay times of the active layer contain the information about the O2 trapping, and electroluminescence (EL) decay times of the OLEDs contain the information about the device degradation

고성능 유기 다이오드를 위한 유기물/금속 계면 연구

학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2014. 2. 이창희.Organic electronics have received a great attention for next generation electronics due to lots of advantages such as easy patterning, flexibility, light-weight, and potential of large-area application. In spite of such a lot of advantages, however, there are still many issues that need to be improved such as charge injection, mobility, lifetime, operational stability, reliability, and uniformity. One of the important issues to make high performance diode is to improve charge injection efficiency. In this thesis, we investigate organic/metal interface of the diode to enhance charge injection efficiency and demonstrate high performance organic diodes. Two major methods are used to improve device performance: improved charge injection by electrical annealing and reduced hole injection barrier by using permanent dipole moment of self-assembled monolayer (SAM). First, we investigate the effect of electrical annealing on pentacene diode to which electrical annealing has not been applied because it cannot have ionic species. By using molybdenum trioxide (MoO3) instead of ionic species, electrical annealing can be applied to thermally deposited device which is advantageous for fabricating high performance devices. After electrical annealing, The turn-on voltage is reduced from approximately 1.3 V to 0.2 V and current at 3 V is increased from approximately 0.2 mA to 1 mA without increase of the reverse-bias current. In addition to MoO3 as a hole injection layer, 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) and copper hexadecafluorophthalocyanine (F16CuPc), which have deep highest occupied molecular orbital (HOMO) levels, show electrical annealing effect but poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), 4,4,4-tris(N-3-methylphenylamino)triphenylamin (m-MTDATA), and copper phthalocyanine (CuPc) do not affect electrical annealing. the cutoff frequency was increased from 10.5 MHz to 85.7 MHz. There is no improvement of current or reduction of turn-on voltage by using thermal annealing only, indicating that electric field plays an important role for electrical annealing. From the time of flight secondary ion mass spectrometry (ToF-SIMS) and impedance spectra, we conclude that the device performance of the pentacene diode is improved by electrical annealing due to the creation of the pentacene:MoO3 mixed layer. The mixed layer effectively increases charge injection by reducing small potential barrier which causes the turn-on voltage of current–voltage (I–V) characteristics and the RC-component at Au/MoO3/pentacene interface of impedance spectra. Note that because this uniform and thin pentacene:MoO3 mixed layer cannot be formed by thermal evaporation, electrical annealing is the best technique to form the uniform, thin, and gradual pentacene:MoO3 mixed layer for improving device performance. After electrical annealing, Al penetration into the pentacene layer was also observed. Because Al was deposited on the polycrystalline pentacene, Al spikes are formed at the pentacene grain boundary. These Al spikes can induce a higher electric field, facilitating the penetration of Al. Therefore, the penetrated Al may create rod-like structures that can be modeled as constant phase element (CPE). Second, we investigated the structure–property relationship of pentacene on gold and SAM-treated gold along the vertical direction. From the photoelectron spectrometer, the work function of gold, thiophenol (TP)-modified gold and pentafluorobenzenethiol (PFBT)-modified gold is measured to be 4.78, 4.67, and 5.02 eV, respectively. PFBT-treated gold effectively lower the injection barrier between the anode and the active layer, the forward-bias current density of the diode with PFBT-treated gold is much higher than that with pristine gold and finally current density of 100 A/cm2 is obtained at 3 V. In addition, the rectification ratio of the diode is founded to be 7.47 × 105 at 1 V, and 1.05 × 107 at 2.8 V. The 3-dB frequency, in terms of voltage, of the rectifier which is composed of the diode and a capacitor is obtained to be 1.24 GHz. Finally, Vout of 3.8 V at 1 GHz is obtained when input voltage of 10 V is applied. From the X-ray diffraction (XRD), atomic force microscope (AFM), and Raman analysis, pentacene molecules on gold exhibit lying-down orientation and those on PFBT-treated gold exhibit standing-up orientation. These structure differences change the electrical property. The mobility, calculated by space charge limited current (SCLC), of the pentacene film on gold and PFBT-treated gold is measured to be 6.82 × 10-4 and 0.114 cm2V-1s-1, respectively. The XRD patterns and vertical scanning electron microscope (SEM) images show that pentacene on gold exhibits the entangled and disordered structure whereas pentacene on PFBT-treated gold exhibits dense and ordered structure. This poor molecular ordering for pentacene on gold can limit charge transport property, resulting that the mobility of the pentacene film on gold is smaller than that of on PFBT-treated gold.Abstract i Contents v List of Figures xi List of Tables xvii Chapter 1 Introduction 1 1.1 Improved Charge Injection by Electrical Annealing 6 1.2 Improved Charge Injection by Self-Assembled Monolayer 9 1.3 Outline of Thesis 11 Chapter 2 Experimental Methods 13 2.1 Materials 13 2.1.1 Chemical Structures of Organic Materials 13 2.1.2 Preparation of SAMs Solutions 15 2.2 Device Fabrication Methods 16 2.2.1 Preparation of Pentacene Diodes for Electrical Annealing 16 2.2.2 Preparation of Pentacene Diodes with SAM-treated Gold 18 2.2.3 Measurement Setup of Pentacene Rectifiers 20 2.3 Device Characterization Methods 21 2.3.1 Carrier Transport in Organic Semiconductors 21 2.3.2 I–V characteristics measurement 23 2.3.3 Frequency Response of Pentacene Rectifiers 24 2.3.4 Mobility Measurements 26 2.3.5 Impedance Spectroscopy 28 2.3.6 ToF-SIMS Measurement 29 2.3.7 Raman Spectroscopy 29 2.3.8 Other Characterization Methods 30 Chapter 3 Improved Injection Efficiency of Organic Diodes by Electrical Annealing 31 3.1 Systematic Investigation into Improved Device Performance of Pentacene Diodes after Electrical Annealing 33 3.1.1 I–V Characteristics of Pentacene Diodes Applying Constant Voltage or Constant Current 33 3.1.2 Current Characteristics of Pentacene Diodes with Electrical Annealing 35 3.1.3 I–V Characteristics of Pentacene Diodes with Various HILs 39 3.1.4 Frequency Characteristics of Pentacene Rectifiers 42 3.1.5 Thermal Annealing Effect on Pentacene Diodes 44 3.1.6 Hysteresis of the Pentacene Diodes before and after Electrical Annealing 45 3.2 Investigation into Proper Mechanism of Electrical Annealing 46 3.2.1 ToF-SIMS Measurements 46 3.2.2 Pentacene Diodes with MoO3 Doped Pentacene as a HIL 48 3.2.3 Impedance Spectroscopy 53 3.3 Summary 61 Chapter 4 High Performance Pentacene Diodes based on SAM-treated Gold 63 4.1 Effective Work Function of SAM-treated Gold 65 4.1.1 Photoelectron Spectrometer Measurement 65 4.2 Structural study of pentacene on SAM-treated gold 67 4.2.1 Morphological Study Using AFM 67 4.2.2 XRD Analysis 70 4.2.3 DFT Simulations of Single Pentacene Molecule for Raman Spectroscopy 71 4.2.4 Raman Spectra of Pentacene on SAM-treated Gold 74 4.2.5 Mobility of Pentacene on SAM-treated Gold Extracted by SCLC 80 4.2.6 Mobility of Pentacene on SAM-treated Gold Extracted by Photo-CELIV 83 4.2.7 SEM Images of Pentacene on SAM-treated Gold 85 4.3 Electrical Performance of Pentacene Diodes with SAM-treated Gold 89 4.3.1 J–V Characteristics of Pentacene Diodes with SAM-treated Gold 89 4.3.2 Frequency Performances of Pentacene Rectifiers with SAM-treated Gold 92 4.4 Demonstration of UHF Operating Pentacene Rectifiers 95 4.4.1 Design of Antennas for UHF RFID tags 95 4.4.2 Loop Antenna Fabrication Using Screen Printing 97 4.4.3 Demonstration of Pentacene Rectifiers Operating at 500 MHz 99 4.5 Summary 101 Chapter 5 Conclusion 103 Bibilography 107 Publication 117 Abstract in Korean 123Docto

Controlling Thin-film Morphology and Incorporating Novel Semiconducting Molecules toward High Performance Organic Optoelectronic Devices

Organic optoelectronic devices have been widely used in display, energy-storage, and consumer electronics. Insightful understanding on material properties, device architecture, and fabrication processes is inevitable to improve the performance of organic optoelectronic devices. My PhD research focuses on improving the performance of organic photovoltaics (OPV) and organic light-emitting diode (OLED) through the systematic processing and material design. The first part of the dissertation describes how to construct a highly conductive morphology of mixed donor:acceptor heterojunction. Organic vapor phase deposition (OVPD) was utilized to enhance crystallinity of C70 acceptor in the mixed tetraphenyldibenzoperiflanthen (DBP):C70 thin-film. Forming the face-center-cubic (fcc) structure of C70 facilitated charge extraction, thereby improving fill factor (FF) of the corresponding OPVs. The second part presents the study on the morphological stability and reliability of OPVs. The cathode buffer, bathophenanthroline (BCP), undergoes significant morphological degradation. This morphological degradation was successfully suppressed by making the underlying DBP:C70 layer rougher via the moving N2 carrier gas in OVPD. The open-circuit voltage (Voc) of the obtained heterojunction OPVs of DBP:C70 grown by OVPD experienced a negligible drop (< 3 % change) while the equivalent OPVs grown by VTE showed a significant decrease in Voc from 0.91±0.01 V to 0.74±0.01 after 1 Sun illumination for 250 h. The third part explains a more precise way to control the morphology of organic mixed layer. It was found that increase in the growth pressure of OVPD induced reorganization of molecules to form the equilibrium morphology. The morphology of the electron-filtering buffer layer of 3,5,3′,5′-tetra(m-pyrid-3-yl)phenyl[1,1′]biphenyl (BP4mPy):C60 was optimized to achieve the highest electron mobility by means of the control of the growth pressure. Consequently, the resulting OPVs with optimized BP4mPy:C60 buffer showed FF = 0.65±0.01 and a much higher PCE = 8.0±0.2 % compared to PCE = 6.6±0.2 % of the equivalent OPVs with the same composition buffer layer grown by VTE. The fourth part summarizes the effects of the inclusion of novel block-copolymers on the performance of the polymer bulk-heterojunction photovoltaic cells. The block-copolymers were composed of thiophene units with and without a dangling phenyl-C61-butyric acid methyl ester (PCBM) side chain. The added copolymer into the poly(3-hexylthiophene) (P3HT): PCBM active layer resulted in greatly improved thermal stability of P3HT:PCBM. Furthermore, electron conductivity also increased since the fullerene units of the copolymers contribute to the formation of a percolation pathway for electron transport. While PCE of conventional P3HT:PCBM bulk-heterojunction solar cells decreases significantly from 2.6±0.2 to 1.2±0.2% after 90-min of thermal annealing, the equivalent OPVs with the copolymer shows a much smaller decrease in PCE from 3.1±0.2% to 2.7±0.2%. The last section of this dissertation covers the design of phosphorescent OLED employing a metal-free purely organic phosphor. Owing to their much longer triplet lifetime in the millisecond regime compared to microseconds of organometallics, a more careful consideration should be given in the device design. The requirements for the host materials in metal-free purely organic phosphor OLEDs are identified to be a high triplet energy, suitable HOMO and LUMO energy levels, and large spectral overlap with the absorption of the phosphors. Systematic investigation on various host molecules, electron transporting molecules, and the layer thickness of each layer allows us to demonstrate an optimized phosphorescent OLED having an external quantum efficiency (EQE) of 2.5 % at 1 mA/cm2.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/144195/1/bssong_1.pd